Iran's team said it received a good reception to its new plan to make headway in the dispute with global players, who fear Tehran's atomic programme is a disguised effort to build a nuclear bomb, a claim it denies.
"The session focused on technical aspects of Iran's proposal, many questions were put forward," an Iranian source close to the talks told AFP after the first day of discussions wrapped up in Geneva.
"The atmosphere of talks was positive and constructive."
Iran's two-day meeting with the European Union-chaired P5+1 group - the United States, Britain, France, China and Russia, plus Germany - ends a six-month freeze sparked by its refusal to curb uranium enrichment in exchange for the easing of punishing international sanctions.
"The proposal that we have introduced has the capacity to make a breakthrough," senior Iranian negotiator Abbas Araqchi said, telling reporters it was "very comprehensive" but that all parties had agreed to keep it under wraps.
EU foreign policy chief Catherine Ashton was to meet one-on-one with Iran's Foreign Minister Mohammad Javad Zarif later in the evening to take stock of the first day of negotiations, officials said.
The Geneva talks are the first since President Hassan Rouhani took office in August after conservative Mahmoud Ahmadinejad wrapped up his second four-year term in power.
Rouhani, seen as more moderate, has pledged transparency on the nuclear programme and engagement with the international community to try to get the sanctions lifted.
In what Western officials said was a sign of the new mood, the Iranian team on Tuesday delivered a presentation in English for the first time.
Earlier, Zarif said Tehran's plan contained three steps that could settle the long-running nuclear standoff "within a year", with the first achievable "within a month or two, or even less".
EU spokesman Michael Mann said discussions had been "very detailed" and technical, and underlined the "very different" atmosphere compared to previous talks.
A senior US State Department official added: "For the first time, we had very detailed technical discussions, which carried on this afternoon. We will continue these discussions tomorrow."
Iran's Araqchi also praised the "very positive environment" and said the "reaction was good" to Iran's hour-long PowerPoint presentation.
Despite the upbeat tone, Iranian state news agency IRNA quoted Araqchi as saying that snap inspections of the Islamic republic's nuclear facilities were not part of the new proposal.
"It does not exist in the offer," Araqchi told IRNA.
Iran has drawn other red lines, saying it will not accept any demand to suspend uranium enrichment or ship out stockpiles of purified material.
Iran rejects charges that it is developing an atomic bomb, insisting its nuclear programme is for power production alone, and says it wants to settle the issue in good faith.
'We are not here to waste our time'
"We are very serious. We are not here symbolically, to waste our time. We are serious for target-oriented negotiations," Araqchi told reporters in Geneva.
Israel - believed to be the Middle East's only nuclear armed state - warned today against accepting "cosmetic concessions" that would not impede Iran's weapons quest.
It has not ruled out a military strike on archfoe Iran to halt the nuclear drive, and has warned the world not to fall for Rouhani's "sweet talk".
Western negotiators insist they are cautiously hopeful but not naive.
"We have come here with a sense of cautious optimism and a great sense of determination because we believe it's really time now for tangible results," Mann said.
"There are signals from Tehran that they want to engage in these negotiations, that they want to be more transparent. The proof would be if they made real progress," he said.
"We are on our side ambitious to move forward quickly... The ball remains in their court."
A senior US administration official said earlier in Geneva that any easing of sanctions would be "targeted, proportional to what Iran puts on the table".
"We are hopeful, but that has to be tested with concrete, verifiable actions," the official said.
A first meeting between Zarif and his counterparts from the six powers took place last month during the UN General Assembly, accompanied by a landmark two-way meeting with US Secretary of State John Kerry.
Kerry underlined Sunday that while the diplomatic window was "cracking open", Washington was serious about never allowing room for a nuclear-armed Iran.